Here's a link to a new study that suggests that ivy growing on walls may be better than street trees in cleaning the air in "street canyon" situations. (Street canyons are roads surrounded by tall buildings, where air tends to linger.)
Here's more from the website:
"The researchers ran computer simulations to determine how green walls and roofs might affect pollutant concentrations at street level. Adding plants to walls would cut nitrogen dioxide levels by 15 percent and small particulate matter by 23 percent, the authors estimate. In areas with little wind, those numbers could reach 40 and 60 percent. Green roofs didn’t perform as well because they don’t directly affect the air near the street.
Trees also help clean the air, but they can keep street-level air from mixing with the air above. At low to medium pollutant levels, planting trees will still reduce air pollution, the team predicts. If a city is very polluted, however, trees could actually increase nitrogen dioxide levels near the street."
Read more here: http://www.conservationmagazine.org/2012/09/breathing-walls/